Castile soap is a traditional, plant-oil-based cleanser, originally made with pure olive oil, which gives it its name from the Castile region of Spain. It is classified as a “true soap” because it is created through saponification, a chemical process. Unlike modern synthetic detergents, this process naturally results in a product with a high pH. This high alkalinity defines the soap’s performance and determines how it interacts with the surfaces it cleans, often requiring users to adjust their routines to manage its effects.
The Alkaline Nature of Castile Soap
The pH of Castile soap, in its undiluted, natural state, is strongly alkaline, typically measuring between 9.0 and 10.5 on the pH scale. This high alkalinity is an unavoidable consequence of its traditional manufacturing method, known as saponification. Saponification is the chemical reaction between a fatty acid source, such as olive or coconut oil, and a strong alkali, which is usually potassium hydroxide (lye) for liquid soaps.
This reaction transforms the oils into soap molecules—salts of the fatty acids—and glycerin, yielding a product that is inherently basic. The high pH is not an added ingredient but a direct result of the chemistry involved in creating a true soap. All true soaps, by definition of their creation method, will exhibit this elevated pH level.
The precise final pH is dependent on the specific oils used and the exact formulation, though it always remains significantly above the neutral pH of 7. For instance, soaps made with a high percentage of coconut oil may trend toward the higher end of the range. The resulting alkalinity is what gives Castile soap its powerful cleaning ability, as alkaline solutions are highly effective at breaking down grease and oil.
Impact on the Skin’s Acid Mantle
The skin maintains a naturally protective layer known as the acid mantle, which helps regulate the skin’s moisture and serves as a defense against environmental irritants and pathogenic microorganisms. This mantle is slightly acidic, with a typical pH ranging from 4.5 to 5.9 across most healthy individuals. This low pH is maintained by sweat, sebum, and the activity of resident microflora.
When the skin is washed with a highly alkaline product like Castile soap, the acid mantle is temporarily disrupted, causing the skin’s surface pH to rise significantly. This shift in the pH balance can persist for several hours after washing, which may lead to measurable changes in the skin barrier function. The elevated pH can damage the lipids in the stratum corneum, increasing transepidermal water loss and causing feelings of dryness or tightness.
For individuals with already compromised skin barriers, this disruption is particularly relevant. Conditions such as atopic dermatitis or eczema are often associated with a higher-than-normal skin pH, and using alkaline cleansers can worsen dryness, itching, and flare-ups. The bacteria implicated in acne also thrive in less acidic environments, meaning high-pH soap can unintentionally alter the skin’s microbiome in a way that favors these harmful organisms.
The rise in alkalinity also interferes with the optimal function of certain enzymes responsible for lipid production and barrier repair, which operate best in an acidic environment. Although the skin restores its acid mantle over time, the repeated use of a high-pH product creates a cumulative stress on the barrier. This makes the skin less resilient, more susceptible to irritation, and less capable of retaining moisture effectively.
Modifying Castile Soap for Specific Uses
Because of its strong alkalinity, users often need to employ specific techniques to manage the effects of Castile soap, especially for personal care. For general household cleaning, heavy dilution is the most common modification, which reduces the soap’s concentration and its residual effect on surfaces without changing its inherent pH. For instance, only small amounts of the liquid soap are needed for tasks like mopping or dishwashing.
When used as a body or hair wash, the high pH can cause hair cuticles to lift, resulting in a matted or tangly texture and dull appearance. To counteract this, users frequently follow a Castile soap wash with an acidic rinse, such as a highly diluted solution of apple cider vinegar or lemon juice. The acidic rinse works to quickly restore a lower pH to the surface of the hair or skin, tamping down the cuticles for a smoother feel.
This two-step process—alkaline wash followed by an acidic rinse—is a deliberate strategy to utilize the powerful cleansing action of the soap while mitigating its high-pH effects. The acidic rinse neutralizes any soap residue that may react with minerals in hard water, which often leaves a waxy film on hair and fabric. Dilution ratios for these rinses, such as a 1:1 mixture of water and apple cider vinegar, are carefully chosen to ensure the surface pH is successfully lowered.